Back in the day, when we were teenagers and played D&D almost every day, my favourite games to play were those around levels 4-7. Reasonably beefy characters who had gathered a few magic items and could go up against a range of foes including dragons, hydras, giants, etc., and yet could be (and often were) brought to ruin by level-draining undead or a failed save versus poison.

Nowadays, I find the brutal, hardscrabble, survival horror of 1st-level adventures, in which a single spear-thrust can kill and resource management is key, to be strangely satisfying, and I have renewed respect for the triumphant achievement of a character gaining 2nd level. That said, I'm still pleased when the characters gain a couple of levels of "cushioning" and their tactical options begin to expand. At 5th level, magic-users can start casting spells like Dispel Magic, Fireball, Fly, or Water Breathing, all of which are (literally) game-changing. When I have ideas for adventures, they tend to be suitable for 3rd-5th level characters, say around the 10,000 XP mark.

Back in the day, level 12+ was not uncommon, but not now. My recent WB campaign had PCs of 4th-5th level, which seemed a good balance of power and risk. Getting past that takes a lot longer anyway, due to the doubling XP requirements for most classes, so that seems like a good spot to hangout for a while. That said, I also love low-level play, both as a player and DM, and our group does mostly that lately.

I recall you saying in another thread that you award 100 XP per HD for monsters defeated, rather than the nonlinear scale introduced in Supplement I: Greyhawk (a variant of which is in every edition of S&W). That will have the effect of speeding play at low levels and slowing it at higher levels, which seems highly desirable to me, so I intend to use that system in future, at least when playing White Box.

Perhaps this was an influence on John Stater's Bloody Basic game, of which there are several variants, but maximum level is 6 in all of them. (In the "classic edition", a 6th-level fighter is a Grognard. I've mentioned elsewhere my dislike of level titles, but that one amuses me.)